It wasn't immediately clear whether the lockdown was related to a drill or an actual threat

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Authorities block an entrance to Redstone Arsenal, June 27, 2017, in Huntsville, Ala. The military post said in a tweet it was on lockdown Tuesday amid reports of possible active shooter, telling workers to "run hide fight."

Authorities locked down an Alabama military post on Tuesday after two 911 calls were made about a potential active shooter, but no shots were fired and officials were investigating the possibility of a hoax.

The lockdown created confusion and worried many people with friends and family at the Redstone Arsenal installation near Huntsville, where more than 30,000 government employees, civilians and contractors work daily. After a tweet advised workers of a possible active shooter and told them to "run hide fight" many people took to social media to offer their prayers and await updates.

Col. Tom Holliday, the garrison commander, said most of the base was cleared about two hours after the initial 911 calls.

"We did have initial reports of someone hearing shots fired and someone seeing a weapon. That in itself drove us to our response," he said. "I would say that the situation that we live in in the United States today leads to caution. Honestly, if we have to overreact, I would rather overreact than underreact."

He said an active shooter drill planned for Wednesday would not take place because officials learned during the lockdown that the system worked as designed.

In a tweet announcing the possible shooter, the base said: "Possible active shooter on the Arsenal. Installation is locked down. Run hide fight."

Police cars with flashing lights blocked entrances to Redstone, which also has facilities used by federal law enforcement agencies. Images on social media showed an ambulance and what appeared to be civilians standing outside the Sparkman Center, which includes multiple buildings with offices, an auditorium, cafeteria and an interior courtyard.

Amy Jay, whose husband was working at the Sparkman Center, said he was told the situation was over through an announcement over the building's public address system.

"Feeling like I can breathe again!" Jay wrote in a message to The Associated Press. "My husband is a contractor and a veteran so when we hear things like this we're reminded of what happened at Fort Hood," she said, referring to the 2009 shooting at a military base in Texas.

Redstone is home to Army missile defense programs and other military offices. NASA's Marshall Space Flight Center, which oversees rocket propulsion operations, also is located at Redstone.